Simple statements

The only kind of simple statement is an expression evaluated for its side
effects.
Every simple statement must be terminated with a semicolon, unless it is the
final statement in a block, in which case the semicolon is optional.
(Semicolon is still encouraged there if the block takes up more than one line).

Any simple statement may optionally be followed by a
single modifier, just before the terminating semicolon.
The possible modifiers are:

The
if
and
unless
modifiers have the expected semantics.
The
while
and
until
modifiers also have the expected semantics (conditional evaluated first),
except when applied to a do-BLOCK or a do-SUBROUTINE command,
in which case the block executes once before the conditional is evaluated.
This is so that you can write loops like: